All sides agree that the tentative agreement to trim 5 instructional days in 2012-13 is a bad outcome for students, but some defend it as a necessity.

A tentative agreement to shorten the school year for Los Angeles students — for the fourth consecutive year — is almost certain to weaken academic gains, and was driven, critics said, by expediency more than the best interests of students.

The deal reached last week between L.A. Unified and its teachers union calls for canceling up to five instructional days from the 2012-13 school year. It also could reduce teacher pay by the equivalent of 10 days overall, about a 5% salary cut. This would bring to 18 the number of school days cut over four years.

All sides agree that the pact is bad for students but some insist it was unavoidable. The district had come under increasing pressure to avoid eliminating adult education and elementary arts programs and sharply increasing class sizes, among other things. The union wanted to spare more than 4,000 teachers and others from layoffs, although it still stands to lose more than 1,300 members.

The shorter school year comes as student achievement already is threatened by larger classes, diminished counseling, slashed summer school and fewer resources to maintain clean, orderly campuses. At the same time, L.A. Unified is under pressure to boost test scores, and use them as part of teacher evaluations.

"We seem to be shrinking everything for the children of Los Angeles," said United Teachers Los Angeles President Warren Fletcher. "This is not fair. We want a full school year, fully staffed. That's what's best for kids."

The agreement prevented even more drastic action: the Board of Education had planned to vote Tuesday on issuing final layoff notices.

Instead, the school board approved a reduced school year linked to the one-year pay cuts, and the outlook doesn't appear much better for the following year. Democratic lawmakers in Sacramento recommended legislation this week that would allow districts to cut up to three weeks off the next two school years — on top of the five days already approved, if voters fail to approve a tax initiative on the November ballot.

Dozens of school districts throughout the state have trimmed the school calendar to reduce layoffs and other deep program cuts. The legal authority to shorten the school year was granted by the governor and Legislature.

In the 2008-09 school year, 98% of California districts maintained a 180-day calendar. By 2010-11, that figure had dropped to 61%. And 20% of districts, including L.A. Unified, had dropped to the minimum 175 days, according to the state's legislative analyst's office.

"For low-income students and English learners who often enter school already behind their peers, shortening the school year is truly the unkindest cut," said Arun Ramanathan, executive director of Education Trust-West, an Oakland-based advocacy group. "These are tough times, but districts have an obligation to prioritize the needs of their students."

Some critics accuse officials of taking the path of least resistance — under pressure from influential teacher unions.

There is, in fact, a strategic advantage for unions in taking furlough days and shortening the school year. The salary cuts that result are temporary; they expire after one year and must be renegotiated every year.

In the process, teachers avoid making permanent concessions on pension or health benefits. L.A. Unified employees still pay no monthly premiums for health insurance for themselves or family members. And teachers still receive raises based on experience or additional education.

At the same time, trimming the school year could generate the outrage needed to build public support for boosting state funding, said Dan Schnur, director of the Jesse Unruh Institute of Politics at USC.

"Shortening the school year sets off alarm bells," Schnur said. "You're not going to mobilize nearly as many people by warning them about the need to renegotiate pension and health benefits."

The alarm is justified in the view of many advocates. A 2007 study by University of Maryland researchers looked at local schools that failed to meet third-grade improvement targets. More than half would have met goals if those schools hadn't been closed an average of five days a year because of bad weather — based on a comparison with other schools and on achievement in years with better weather.

Separate studies in 2008 and 2011 found that a significantly longer school year was a key factor in boosting student achievement at New York City charter schools.

Los Angeles teachers have sustained a succession of one-year pay cuts, tied to a shorter work year, that ranged from about 2% to 5% annually. Even so, nearly 8,000 district employees have lost jobs; a small number have been rehired. Those ranks will swell further because of declining enrollment, lost state and federal funds, and already approved cuts to adult schools and other programs.